The objectives of this research are to investigate the mechanisms by which serum proteins are transported into urine in the normal and in the natural occurring and experimental disease state. The amount of protein in the proximal and distal tubules will be measured by radioimmunoassay methods on samples collected by nephron puncture. Studies will be done in control rats and in rats made proteinuric by the injection of the aminonucleoside of puromycin. To provide another approach to the problem immunofluorescence microscopy will also be done and the findings correlated with nephron puncture findings. Using a series of albumins and albumin derivatives in normal and nephrotic rats and following the relative rate of excretion it should be possible to isolate the factors influencing selective reabsorption from those responsible for permeability.